But the focus on anti-vaccine content on social media can obscure the most important factor in whether children get vaccinated: the rules in their home states, which are being revisited in legislative debates across the country that have received far less attention.

A teenager testified before Congress on Tuesday that he got vaccinated in defiance of his mother, who he said got her anti-vaccination views from social media. Although this kind of misinformation can endanger public health, it’s not obvious that social media is substantially increasing overall vaccine hesitancy. Despite rapid growth in the proportion of Americans using social media sites, flu vaccination rates and infant immunization levels have largely remainedstable in recent years. Moreover, fears about and resistance to vaccination are not new; they date to the late 18th century, when the first vaccine was developed.

Social media may simply provide a new pretext for hesitant parents who would otherwise cite a different reason for their decision. In other words, we may be mistakenly treating what is largely a symptom of vaccine hesitancy as its cause — an example of a recurring pattern in which we fault social media for causing problems that it is merely making more visible. (For instance, the internet and social media are often blamed for fueling political polarization, but the trend toward greater polarization long predates social media and is sharpest among older people, the group least likely to use new technology.)

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A Pinterest office in San Francisco, The social media platform recently curbed vaccine search results to help stop the spread of misinformation.CreditAnastasiia Sapon for The New York Times

It seems reasonable to ask that social media companies avoid worsening the global problem of vaccine hesitancy. But a more immediate threat in the United States are the state policies that make it easy to avoid immunization requirements for children entering kindergarten. Emory University’s Saad B. Omer and his colleagues have found that broad philosophical and religious exemptions to vaccine requirements — a set of policies that expanded before the advent of social media — are associated with substantially higher rates of unvaccinated children. They found this was particularly true during the 2005-2006 to 2010-2011 period, when exemptions grew fastest in these states.

Reversing such policies can increase vaccination rates. After a measles outbreak originating in Disneyland, for instance, California eliminated nonmedical exemptions from vaccination requirements for kindergarten entry and tightened compliance requirements. Although the process provoked controversy and required a bill to be passed along partisan lines (a risky approach), vaccination levels increased afterward — a public health success. For this reason, the American Academy of Pediatrics supports eliminating all nonmedical exemptions.

Debates over these issues are taking place now in a number of states. Most center on proposals that would restrict or eliminate nonmedical exemptions (as in Washington). But outside of the national spotlight, some states are considering proposals to expand exemptions or are elevating fringe views. The Arizona legislature, for instance, passed a bill creating a religious exemption to vaccination mandates (though almost no religions oppose vaccines), with one lawmaker calling such requirements “Communist.”

Similarly, a state legislator in Texas who supports a pending bill making exemptions easier to obtain said people weren’t dying of measles in America because of “antibiotics and that kind of stuff.”

Such misinformation can have a great deal of influence on public health when it has the force of law behind it — far more than social media alone.

Brendan Nyhan is a professor of public policy at the Ford School at the University of Michigan. Follow him on Twitter at @BrendanNyhan.